1. Field of the Invention
The present invention relates to an organic light emitting display, and more particularly, to an organic light emitting display that can minimize degradation of a drive transistor in a pixel circuit of the organic light emitting display and brightness change of an organic light emitting diode caused by the degradation.
2. Description of the Related Art
Generally, a conventional organic light emitting display is a display device emitting light by electrically exciting fluorescent or phosphorescent materials. The organic light emitting display displays images by driving N×M number of organic light emitting cells. The organic light emitting cell includes an anode (ITO), an organic thin film and a cathode (metal) as shown in FIG. 1. The organic thin film has a multi-layered structure including an emitting layer (EML), an electron transport layer (ETL) and a hole transport layer (HTL) for improving lighting efficiency with a balanced combination of an electron and a hole. Further, the organic thin film may include separate an electron injecting layer (EIL) and a hole injection layer (HIL).
A technique for driving the organic light emitting cell includes a passive matrix (PM) technique, and an active matrix (AM) technique using a thin film transistor (TFT) or a metal oxide silicon thin film transistor (MOSFET). The passive matrix technique drives a light emitting cell by forming an anode to be intersected with a cathode and selecting a line. The active matrix technique is a driving technique that connects the transistor and the capacitor to respective indium tin oxide (ITO) pixel electrodes to maintain a voltage by a capacity of a capacitor.
The transistor used in this active matrix technique is an amorphous silicon thin film transistor or polycrystalline silicon thin film transistor. When the amorphous silicon thin film transistor is used as a drive element, current driving power is relatively low. However, uniformity of the display is excellent and the transistor is advantageous in large scale process. However, a threshold voltage of the drive transistor in the pixel circuit for flowing current is gradually increased because the silicon structure is damaged according to current flow caused by a voltage applied to a control electrode thereof. As shown in a transistor current equation of the following equation 1, an amount of current applied to the organic light emitting diode is decreased according to the increase of the threshold voltage. Accordingly, brightness of each pixel is deceased. Thus, there is a problem that brightness of the organic light emitting display is gradually decreased with the lapse of time.
Moreover, there is another problem that the threshold voltage degradation of each pixel circuit is changed according to a data voltage that has been already applied to each pixel circuit, the entire brightness of the organic light emitting display becomes non-uniform.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                                                      ⁢                              I            OLED                    =                                    β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                                  1      
In the above equation 1, IOLED is a current flowing in a drive transistor and an organic light emitting diode, VGS is a voltage between gate and source of the drive transistor, VTH is a threshold voltage of the drive transistor, and β is an electric conductivity of the drive transistor.